In a wideband voltage controlled oscillator for use in a two-way radio (e.g., in the range of 70-80 MHz bandwidth), it is typical for the drive current, and hence output power of the oscillator circuit to increase as the frequency of operation of the VCO increases. In a typical wideband Colpitts VCO circuit, the input current variation can be as much as 1 mA, and power output can vary by approximately 3 dB. These variations can be detrimental where the radio specifications do not allow for them. For example, where there is a great need to conserve current when the VCO is in the receive (Rx) mode in order to minimize the overall current drain of the radio. The problem with output variations is that a large VCO output variation will drive the buffer stage that is coupled to the output of the VCO differently, over variations in frequency and hence affect the gain and current drain of the buffer stage as well.
The overall sideband noise ratio (SBNR) of the message transmitted by the two-way radio is also affected by current variations (e.g., variations greater than 0.5 ma) in the VCO. These current variations can have significant effects on the flatness of the SBNR curve over frequency. This is mainly due to the dependency of the SBNR on the power feedback to the VCO transistor and in the transistor noise figure, which varies over current. Also, the lower the feedback resistance (see Rf 116 in FIG. 1) in the feedback loop (e.g., Rf&lt;50 ohms), the greater the sensitivity of the SBNR to variations in current. In typical Colpitts VCO circuits SBNR becomes a problem since Rf is typically a low resistance.
In FIG. 1, a conventional Colpitts VCO 100 is shown. Oscillator 100 comprises several sections which include: a conventional electronic tank section 102, feedback section 106 including active element 108, and a biasing section 104. Some of the key parts of oscillator 100 that will be of interest, due to their relationship to the present invention, are found in feedback section 106 and include: feedback resistor (Rf) 116, capacitor (C1) 112, capacitor (C2) 110, capacitor (C3) 114, inductor 122 and active device (transistor) 108. VCO 100 further includes an input control voltage terminal 118, a negative input control voltage (-VEE), a positive bias voltage input (Vcc) and a VCO output port 120, which is subsequently coupled to a buffer stage (not shown). NPN transistor (gain element) 108 includes a control terminal which is the base of the transistor, a first terminal which is the collector and a second terminal which is the emitter of the transistor.
In FIG. 2 an AC model of the VCO circuit of FIG. 1 is shown. FIG. 2 takes away all DC components associated with FIG. 1. Varactor (Cv) 202 is the equivalent tuning capacitor of the varactor tank 102. FIG. 2 also shows capacitor (C1) 208, capacitor (C2) 210, capacitor (C3) 206, feedback resistor (Rf) 216, transistor (active element) 212, inductor (L) 204 and VCO load (RL) 214.
For analysis, FIG. 2 can be transformed into an open loop AC model as shown in FIG. 3 which utilizes the hybrid-.pi. transistor model. Where (V1) is the feedback voltage at the base of the transistor, (V2) is the feedback network input voltage and section 302 is the equivalent inductance of the tuning network. A further simplification of the open loop model of FIG. 3, is an amplifier having an amplifier gain of "A", in series with a feedback network having a feedback ratio of ".beta." coupled to the output of the amplifier. The amplifier having an input voltage of (V1 as shown in FIG. 3) and the feedback network having an input voltage of (V2).
The feedback ratio for FIG. 3 can be calculated as follows, ##EQU1## where L3 equals the equivalent inductance of tuning network 302. ##EQU2## where ##EQU3## therefore ##EQU4## (equation number 1). An electrical analysis (possibly utilizing any one of many commercially available circuit analysis programs) of the hybrid-.pi. open loop model of FIG. 3 shows that the open loop gain increases as the frequency of operation of the VCO increases. As a result, the current drain of the VCO increases with frequency. A need therefore exists for a VCO having fairly level current drain, as well as lower SBNR and fairly constant power output over a wide operating frequency range in order to avoid the current, power, and SBNR variations associated with conventional wideband oscillators.